Amide-I and amide-A bands of δ-valerolactam and its dimers: A combined matrix isolation - FTIR spectroscopy and quantum chemistry study

Singly and doubly hydrogen-bonded dimers of δ-valerolactam, a biologically important cyclic amide, are studied by matrix isolation FTIR spectroscopy and quantum chemistry methods. The spectral features of the doubly hydrogen-bonded dimer (DHBD) are nearly similar to those displayed by the dimer produced in CCl4 solution at room temperature. On the other hand, the matrix environment allows identification of singly hydrogen-bonded dimer (SHBD) which is a transient species in the solution and gas phases. Identification of the two species are made by noting the spectral shifts of amide-I and amide-A bands corresponding to the two dimers and observed changes are consistent with predictions of electronic structure theory calculations. A cooperative strengthening of hydrogen bond is manifested in the large increment of CO and NH bond lengths in DHBD, as compared to those in SHBD.